<apex:page id="thepage" standardController="Opportunity" extensions="OpportunityExt">
 <apex:form id="theform">


        <script type="text/javascript">
        function run(){
        document.getElementById("thepage:theform:GoButton").disabled=true;
        }
        </script>

  <apex:commandButton action="{!Go}" value="Go"
                        id="GoButton" onclick="run();"/>

  </apex:form>


</apex:page>